Steam heated polyester production process

ABSTRACT

Processes for making polyesters in a polyester production facility are disclosed, that include the steps of forming a reaction medium comprising at least one monomer that includes terephthalic acid (TPA) and/or an ester derivative of TPA; subjecting at least a portion of the reaction medium to one or more chemical reactions in the polymer production facility to thereby produce the polyester; and heating the reaction medium at one or more locations in the polyester production facility, wherein at least 50 percent of the total energy input employed for the heating of the reaction medium is provided by indirect heat exchange between the reaction medium and steam.

This application claims priority to U.S. Provisional Application Ser.No. 61/109,983, filed Oct. 31, 2008 the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND

It is known to produce polyester polymers through condensationpolymerization in the melt phase starting from diacids and/or diestersas one type of monomer and diols as the other type of monomer. Inparticular, it is known to form polyester polymers based on aromaticdicarboxylic acids such as terephthalic acid (TPA) as the principaldiacid and ethylene glycol (EG) as the principal diol with the resultingpolymer being called polyester terephthalate (PET) herein. In additionto TPA and EG, PET polymers often contain lesser amounts of othermulti-carboxylic acids, e.g. aromatic acids such as isophthalic acid(IPA) and trimellitic acid (TMA), and other multi-alcohols, e.g.1,4-cyclohexane dimethanol (CHDM) and diethylene glycol (DEG). DEGmonomer may be formed in situ from dimerization of EG. Throughout thisdisclosure and in claims, TPA may be replaced optionally in whole orpart by any of its ester derivatives, including but not limited todimethyl terephthalate (DMT) and bis(2-hydroxyethyl)terephthalate(BHET). Collectively and individually, these various multi-carboxylicacids, multi-esters, and multi-alcohols are termed monomers, as usedherein. As used herein, process materials are monomers, catalysts,polymer additives, reaction medium, PET melt product, and variousbyproducts of the PET formation reactions. Typical byproducts from theformation of PET comprise water, methanol and EG, and variousdegradation byproducts. Typical degradation byproducts from theformation of PET comprise acetaldehyde, DEG, various dioxanes, andvarious colored conjugated aromatic molecules. As used herein, reactionmedium is a mixture comprising at least a portion of EG mixed with atleast a portion of TPA and/or an ester of TPA. Optionally, reactionmedium may comprise various catalysts, polymer additives, and byproductsof the PET formation reactions.

The melt-phase synthesis of PET is typically executed at temperatures inexcess of 250° C. Such high temperatures are needed both to maintain thereaction medium in a flowing molten state and to promote chemicalreaction rates. Generally, the importance of higher temperatureincreases as the degree of polymerization increases.

It is known that the heat of reaction for formation of the ester bondlinkages in PET is only mildly exothermic in comparison to the thermalenergy input needed to elevate the monomers to the temperature of thereaction medium and to remove as vapors the byproducts of thecondensation reaction, e.g. water, methanol. In a commercial-scale PETmelt product synthesis facility, very large amounts of thermal energyare added to process materials, especially to the reaction medium. Verylarge heat-transfer areas are needed to exchange this thermal energythrough conductive, isolating, heat-exchange boundary surfaces, whichtypically comprise various metals and metal alloys.

The large input of thermal energy during synthesis of PET melt productis typically provided by one or more types of heat-transfer fluidthrough conductive, isolating, heat-exchange boundary surfaces. Theconventional choices for heat-transfer fluids are various relativelyhigh molecular weight organic materials that have relatively low vaporpressures even at temperatures in excess of 300° C. This relatively lowvapor pressure, as compared to the vapor pressure of water or of lowmolecular weight organic materials, ameliorates the mechanical designand cost of the conductive, solid boundary surface needed to isolate thereaction mass from the heat-transfer mass while enabling the transfer ofthermal energy. These low-vapor-pressure organic heat-transfer fluidsare used in various combinations of liquid and vaporized forms, yieldingboth sensible heat and latent heat of vaporization to the processmaterials. Suitable low-vapor-pressure organic heat-transfer fluids areavailable in the Dowtherm, Therminol, and other commercial productlines; and Dowtherm A and Therminol 66 are preferred embodiments.

Unfortunately, such low-vapor-pressure organic heat-transfer fluids haveseveral drawbacks. The low-vapor-pressure organic heat-transfer fluidshave relatively low sensible heat capacity and latent heat ofvaporization, especially as compared to water, and thus requirerelatively large mass flow rates to transfer the required amount ofthermal energy into the process materials. Conduit diameters forconveying low-vapor-pressure organic heat-transfer fluids in a large PETmelt product synthesis facility may exceed 0.5 m. The low-vapor-pressureorganic heat-transfer fluids are flammable and require considerablesafety precautions. For example, a “river of fire” can occur when anexchanger tube ruptures inside a fuel-fired furnace where thelow-vapor-pressure organic heat-transfer fluid is typically heated.Despite careful selection of the organic molecules and rigorousminimization of dissolved oxygen therein, the low-vapor-pressure organicheat-transfer fluids are challenged on thermal stability in theexchanger tubes of a fuel-fired furnace where they are typically heated.The low-vapor-pressure organic heat-transfer fluids are expensive, withthe filling inventory for a world-scale PET melt product synthesisfacility costing in excess of one million US dollars. Thus, the combinedcosts for using low-vapor-pressure organic heat-transfer fluids addconsiderably to the cost of manufacturing PET melt product: capital costfor fuel-fired furnace; capital cost for large diameter piping, valves,insulation, controls, and pumps; capital cost for fire protection;circulation pump energy consumption; fluid degradation losses; thermalenergy losses on large pipe sizes, even with thick insulation; andworking capital for fluid inventory.

SUMMARY

In one aspect, the invention relates to processes for making polyestersin a polyester production facility, which include the steps of: forminga reaction medium comprising at least one monomer that includesterephthalic acid (TPA) and/or an ester derivative of TPA; subjecting atleast a portion of the reaction medium to one or more chemical reactionsin the polymer production facility to thereby produce the polyester; and(c) heating the reaction medium at one or more locations in thepolyester production facility, wherein at least 50 percent of the totalenergy input employed for the heating of the reaction medium is providedby indirect heat exchange between the reaction medium and steam.

The inventors have discovered an economical way to use high pressurewater vapor (steam) as the principal heat-transfer fluid in a polyesterproduction facility, such as a PET melt product synthesis facility, atcommercial scale, overcoming the previous economic barriers thatprevented use of steam heating for this application. In certainembodiments, the polyester production facility employs a small diameter,elongated heat-exchange conduits rather than jacketed vessels, jacketedconduits, and larger diameter elongated heat-exchange conduits insidevessels. The small diameter, elongated heat-exchange conduits alloweconomical mechanical construction despite the very much higher pressureof steam as compared to low-vapor-pressure organic heat-transfer fluidsat comparable temperature. In certain embodiments, the polyesterproduction facility uses relatively high superficial flow velocities forprocess materials near conductive, isolating, heat-exchange boundarysurfaces so that the heat transfer rate per unit boundary surface areais increased and the required boundary surface area is thus diminished.

In certain embodiments, the polyester production facility comprises ashell-and-tube exchanger using steam at pressures above 8 megapascals onthe shell-side and with a three-phase reaction medium comprising liquid,solid, and vapor flowing with a superficial velocity above 1.4 metersper second on the tube-side in order to provide the majority of allthermal energy input to a polyester production facility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a polyester production facilityshowing the primary reactants introduced into the facility, the primaryproducts produced from the facility, and two separate furnaces heatingtwo thermal energy mediums (“steam” and “other HTM”) that provide heatto the facility.

FIG. 2 is a more detail depiction of a polyester production facilityshowing a series of reactors (Rx#1-RX#5), with high-pressure steamproviding thermal energy to the initial reactors (Rx#1-Rx#3).

FIG. 3 is a schematic depiction of a polyester production facilitysimilar to the facility of FIG. 1, but employing only a single furnaceto heat steam that is subsequently used to provide thermal energy to thefacility, both directly and through heat exchange with another heattransfer medium (HTM).

FIG. 4 is a schematic depiction of a monomer production facility and apolyester production facility with heat-integration between the twofacilities.

FIG. 5 is a more detailed depiction of the monomer production facilityof FIG. 4 showing the individual steps of monomer production, as well aslocations where heating can be provided by low-pressure steam.

FIG. 6 is a schematic depiction of a system for heat-integrating amonomer production facility and a polyester production facility usingflashed steam and steam-heating of another HTM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors have discovered and disclose herein a process for forminga polyester melt product, such as PET, from polycarboxylic acid andglycol monomers, such as TPA and EG, wherein the thermal energy inputsourced from steam heat-transfer fluid is at least about 30, 50, 70, 80percent of the total thermal energy input to the polyester productionfacility from all heat-transfer fluids. A schematic depiction of anexemplary polyester production facility is provided in FIG. 1. In oneembodiment the polyester production facility is a PET melt productsynthesis facility. FIG. 2 shows a more detailed depiction of apolyester production facility that includes a plurality of seriallyconnected reactors (Rx#1, Rx#2, Rx,#3, Rx#4, and Rx#5). FIG. 2 alsoshows that each of the first three reactors has a shell-and-tube heatexchanger associated therewith, and that the shell-and-tube heatexchangers heat the reaction medium by indirect heat exchange withhigh-pressure steam.

The inventors have discovered that it is preferable that at least about30, 50, 70, 80 percent of the thermal energy input from steamheat-transfer fluid into a polyester production facility is provided bydirect transfer of thermal energy from steam through conductive,isolating, heat-exchange boundary surfaces into reaction medium. This isas distinguished from transferring thermal energy from steamheat-transfer fluid into separated monomers not combined into reactionmedium.

The inventors have discovered a process and preferred apparatus forforming polyester melt product, such as PET, from monomers, such as TPAand EG, wherein the total thermal energy input sourced from steamheat-transfer fluid is at least about 400, 800, 1200, 1400 joules pergram of polyester melt product. More preferably, at least about 350,700, 900, 1200 joules per gram of polyester melt product is exchangeddirectly from steam heat-transfer fluid into reaction medium, asdistinguished from going into separated monomers not combined intoreaction medium.

Preferably the supply pressure of at least about 30, 50, 70, 90 weightpercent of the steam heat-transfer fluid is at least about 2, 5, 6, 7, 9megapascal at the locations where thermal energy is transferred intoprocess materials. In contrast, the supply pressures oflow-vapor-pressure organic heat-transfer fluids used in a polyesterproduction facility are typically less than about 2 megapascals,requiring much less metal thickness for containment.

More preferably, the condensing temperature matching the pressure of thesteam heat-transfer fluid used to heat reaction medium is at least about260, 275, 290, 305 degrees Celsius. Higher condensing temperaturesprovide a larger temperature gradient for exchanging thermal energy intoreaction medium, thus relatively reducing the areas required forconductive, isolating, heat-exchange boundary surfaces.

However, higher condensing temperatures also require greater supply andcontainment pressures for steam heat-transfer fluid. Thus, it ispreferred that the supply and condensing pressures of the steamheat-transfer fluid are limited to less than about 20, 16, 14, 12megapascal for reasons of economy in mechanical design for supplyconduits and vessels and for conductive, isolating, heat-exchangeboundary surfaces.

In contrast to the steam pressure, the pressure of the process materialsis much less, preferably being less than about 1, 0.5, 0.2, 0.1megapascal for the monomers, reaction medium and byproducts. As thedegree of polymerization and molecular weight of the reaction medium areincreased, it is preferred that the pressure of the reaction medium islowered to less than about 0.05, 0.01, 0.001, 0.0001 megapascal.

Also, it is preferred that the temperature of the steam supplied to thepolyester production facility is less than about 600, 500, 400, 350degrees Celsius. At any given pressure, the steam can be far hotter thanits equilibrium condensing temperature. In some applications, such aspower generation, it is highly desirable that the steam supply contain agreat amount of superheat. However, in a polyester production facility,excessive superheat increases costs for metal used for mechanicalcontainment of the steam. Also, the temperature of superheated steam ispotentially damaging to the process materials, especially in films ofprocess materials contacting conductive, isolating, heat-exchangeboundary surfaces, especially during operating excursions with less thandesign flow rates of process materials.

In certain embodiments, the polyester production facility employsconductive, isolating, heat-exchange boundary surfaces that have a smalldiameter. This lowers the cost for mechanical containment of highpressure steam, and it also proves important in maximizing theheat-transfer coefficient on the process side, as is disclosed fartherbelow. For at least about 30, 50, 70, 90 percent of the thermal energytransferred from steam to the polyester production, it is preferred thatthe outside diameter of the conductive, isolating, heat-exchangeboundary surfaces is less than about 0.3, 0.15, 0.06, 0.03, or 0.003meters.

Elongated conduits comprising small diameter, isolating, heat-exchangeboundary surfaces are referred to herein as tubes. The inside of thetubes is referred to herein as tube-side. The outside of the tubes isreferred to herein as shell-side.

In certain embodiments, the polyester production facility providesrelatively high superficial flowing velocities for the reaction mediumin order to promote the amount of thermal energy transferred compared tothe size and cost of the heat-exchange means. In a conventional processfor forming polyester melt product using low-vapor-pressure organicheat-transfer fluids, both sides of the heat-exchange surfaces arecontacted by organic liquids. The thermal conductivities of monomers andreaction medium are roughly comparable to those of typicallow-vapor-pressure organic heat-transfer fluids. Especially when thedegree of polymerization of the reaction medium and its viscosity arerelatively low, the heat-transfer film coefficients are similar forreaction medium and for typical low-vapor-pressure organic heat-transferfluids. In contrast, the heat-exchange properties of condensing steamare far greater. For example, a film coefficient for heat-exchange fromcondensing steam is typically greater than about 6,000 watts per squaremeter per degree Celsius, which contrasts with a film coefficient forheat-exchange from liquid low-vapor-pressure organic heat-transfer fluidthat is typically less than about 2,000 watts per square meter perdegree Celsius. Such a large difference in film coefficient forheat-exchange indicates that the process-side film coefficient, ratherthan the steam-side film coefficient, will largely control the overallheat-exchange rate across the conductive, isolating, heat-exchangeboundary surfaces. Increasing the superficial velocity of the processflows usefully increases the heat-transfer film coefficient on theprocess side of the heat-exchange boundary surface. Increasing theheat-transfer film coefficient on the process side is particularlyvaluable with steam heat-transfer fluid because of the greater thicknessand cost of metal for pressure containment, as compared to alow-vapor-pressure organic heat-transfer fluid.

In one embodiment of the invention, it is preferred to provide asuperficial velocity of process materials, especially reaction medium,that is at least about 0.7, 1.4, 3, 5 meters per second near conductive,isolating, heat-exchange boundary surfaces. It is preferable that theabove superficial velocities are provided within tubes. Superficialvelocity has the usual meaning of time-averaged volumetric flow dividedby the cross sectional area surrounded by the containing and isolatingboundary surface and measured in a plane orthogonal to the direction ofthe time-averaged volumetric flow.

In another embodiment of the invention, it is preferred to provide aliquid and vapor multiphase flow of reaction medium within aheat-exchange means. The vapor phase preferably comprises both EGmonomer and reaction byproducts. In this embodiment, it is preferablethat the volume fraction of vaporized reaction medium is at least about25, 50, 75, 95 volume percent of the total volume of the reaction mediumin the heat-exchange means. It is preferable that the above volumefractions are provided in a portion of reaction medium within or nearthe outlet of reaction medium from a tube in a heat-exchange means.

In another embodiment of the invention, it is preferred to provide aliquid and solid multiphase flow of reaction medium within aheat-exchange means. More preferably, the solid phase comprises apolycarboxylic acid, such as TPA. In this embodiment, it is preferablethat the mass fraction of polycarboxylic acid solid particles is atleast about 2, 4, 6, 8 weight percent of the reaction medium. In thisembodiment, it is also preferable that the mass fraction ofpolycarboxylic acid solid particles is less than about 60, 40, 25, 15weight percent of the reaction medium in the heat-exchange means. It ispreferable that the above mass fractions are provided in a portion ofreaction medium within or near the inlet of reaction medium into a tubein a heat-exchange means.

In still yet another embodiment of the invention, it is preferred toprovide a three-phase reaction medium comprising solid TPA particles,vapor components, and liquid components within the heat-exchange means.The above preferred volume fractions of vapor and the above preferredmass fractions of solid apply in all physically realizable combinationsfor this three-phase mixture.

The inventors have discovered that a preferred embodiment of theinvention places the steam heat-transfer fluid on the shell side and theprocess materials on the tube side. Indeed, this is a preferredembodiment when the process material is specifically reaction medium,more specifically when the reaction medium comprises TPA solid, and mostspecifically when the reaction medium is a three-phase mixturecomprising solid TPA, vapor components, and liquid components.

This arrangement with steam heat-transfer fluid on the shell-siderequires the mechanical design of both the tube-side and shell-side towithstand the pressure of the steam heat-transfer fluid. This causesgreater capital cost than if the lower pressure process material were onthe shell-side. However, the inventors have discovered that placing theprocess material on the tube-side provides very great advantages. First,this disposition provides greater control of the velocity of the processmaterial near the conductive, isolating, heat-exchange boundarysurfaces, thus helping to maximize film coefficients for heat transferin order to reduce overall capital cost. Second, this dispositionprovides greater control of the maximum film temperature of the processmaterial near the conductive, isolating, heat-exchange boundarysurfaces. This can be important when the process material is reactionmedium in order to optimize the balance between desired polymerizationand undesirable side reactions such as dimerization of EG to diethyleneglycol (DEG) and such as formation of colored impurities in thepolyester melt product. Third, this disposition provides greater controlof the residence time and residence time distribution of the processmaterials. Again, this is especially important for reaction medium inorder to optimize the balance between desired polymerization andundesirable side reactions.

It is preferred that the overall heat-exchange coefficient between steamheat-transfer fluid and process materials, especially reaction medium,is at least about 5, 100, 250, 500 watts per square meter per degreeCelsius. This is achieved by selection of steam pressures andtemperatures, tube diameters, process material compositions, processmaterial superficial velocities, and tube metallurgies according to thedisclosures herein. The overall heat-exchange coefficient is based onthe outside diameter of the tubes and is calculated according to methodsand standards of the Heat Transfer Research Institute.

The inventors disclose that certain embodiments of the present inventioncan be particularly useful for the emerging scale of polyesterproduction facilities with capacities from a single facility and or asingle polyester melt product reactor of at least about 100, 200, 400,600 million kilograms per year. Existing steam boilers are alreadyextremely large, with combustion power inputs even greater than onegigawatt. Fired furnaces for heating low-vapor-pressure organicheat-transfer fluids can also be made quite large. However, design of afired furnace for low-vapor-pressure organic heat-transfer fluids mustpay strict attention to film temperatures in order to avoid thermaldegradation of the fluids. This imposes more restrictive constraints onthe firing temperature and on the size and layout of the heater tubes,as compared to a steam boiler. Mass flow rates per watt of thermalenergy content are far less for condensing steam than forlow-vapor-pressure organic heat-transfer fluids. Pipe sizes for thecondensate-water return to a boiler are much smaller than forlow-vapor-pressure organic heat-transfer fluids. Pipe sizes for steamsupply are also smaller, albeit at a higher pressure rating.

The inventors have discovered that certain embodiments of the presentinvention can be particularly useful for providing thermal energy to areaction medium of a single polyester synthesis process facility whereinthe thermal energy thermal input from steam heat-transfer fluid is atleast about 10, 20, 30, 40 megawatts from steam heat-transfer fluid.

The inventors have discovered that certain embodiments of the presentinvention can be particularly useful for providing thermal energy to areaction medium of a single reaction step within a polyester synthesisprocess facility wherein the thermal energy input from steamheat-transfer fluid is at least about 5, 10, 15, 25 megawatts.

The inventors have discovered that certain embodiments of the presentinvention can be particularly useful with shell and tube heat-exchangemeans having tube areas in a single means of at least about 300, 1000,2500, 4000 square meters, based on the outside diameter of the tubes.

The inventors have discovered that certain embodiments of the presentinvention can be useful for exchangers wherein the tubes are made fromany of the many stainless steel alloys, with 304L especially preferred,and the shell is made from any steel alloy, including any stainlesssteel alloy.

As yet another aspect of the invention, the inventors have discoveredthat a preferred method for heating a low-vapor-pressure organicheat-transfer fluid for use in a polyester production facility comprisesusing steam heat-transfer fluid to provide thermal energy to alow-vapor-pressure organic heat-transfer fluid. This is especiallypreferred in a polyester production facility wherein at least a portionof thermal energy is provided to process materials by steamheat-transfer fluid directly through conductive, isolating,heat-exchange boundary surfaces according to at least one disclosureherein. In a polyester production facility, a considerable number ofrelatively low duty heating systems, such as piping and equipment tracercircuits, are needed to provide small thermal energy duties to processmaterials and to offset ambient losses of thermal energy, such as lossesthrough insulation. These low duty heating systems are particularlyrequired for preheating conduits and equipment before admitting the flowof process materials, for when the flows of process materials aretemporarily idled, and for when the flows of process materials arereduced to very low rates. Although steam heat-transfer fluid asdisclosed herein is sufficiently hot for this dispersed, low dutyservice, the very high pressures required cause capital and maintenancecosts to be economically unfavorable for using the steam in the service.Thus, even when larger thermal energy duties for process materials areconverted to steam heat-transfer fluid, there remains a need in apolyester production facility for a thermal energy supply system using alow-vapor pressure organic heat-transfer fluid. For such relatively lowduty heating systems using a low-vapor-pressure organic heat-transferfluid, the inventors have discovered that it is more economical toincrease the size of the fuel-fired steam boiler and to use part of thissteam to heat the low-vapor-pressure organic heat-transfer fluid, ascompared to providing a separate fuel-fired furnace for thelow-vapor-pressure organic heat-transfer fluid.

The advantage of using steam heat-transfer fluid to provide thermalenergy to a low-vapor-pressure organic heat-transfer fluid is especiallypronounced when reliability is considered. The additional complexfailure modes intrinsic with providing at least one fuel-fired furnacefor low-vapor-pressure organic heat-transfer fluid are efficaciouslyreplaced by a simple heat-exchange means, which means typicallycomprises no moving mechanical parts and which does not require controlsand interlocks for combustion safety and environmental emissions.

Thus, the inventors disclose a polyester production facility using steamheat-transfer fluid, according to according to at least one disclosureherein, as the only heat-transfer fluid that receives thermal energydirectly from a fuel-fired furnace. More preferably, the polyesterproduction facility also receives at least a portion of thermal energyfrom another heat-transfer fluid that receives at least a portion ofthermal energy from the fuel-fired steam. Such a configuration isschematically illustrated in FIG. 3. Most preferably, the polyesterproduction facility receives at least a portion of thermal energy from alow-vapor-pressure organic heat-transfer fluid that is heated by thesteam heat-transfer fluid in a heat exchange means comprising tubes. Itis preferred that the low-vapor-pressure organic heat-transfer fluid isheated by steam to a supply bulk temperature of at least about 250, 270,290, 310 degrees Celsius. It is preferred that the low-vapor-pressureorganic heat-transfer fluid is heated by steam to a supply bulktemperature of less than about 400, 360, 340, 330 degrees Celsius. It ispreferred that the maximum film temperature of the low-vapor-pressureorganic heat-transfer fluid is less than about 400, 360, 340, 330degrees Celsius at the conductive, isolating, heat-exchange boundarysurfaces contacted by steam heat-transfer fluid.

The inventors also disclose preferred ranges for the amount thermalenergy provided by a low-vapor-pressure organic heat-transfer fluid thatprovides thermal energy to a polyester production facility. These energyvalues are much lower than in a conventional polyester productionfacility because the major duties for thermal energy are transferred tosteam heat-transfer fluid according to certain embodiments of theinvention disclosed herein. It is preferred that the thermal energysupplied to a polyester production facility by low-vapor-pressureorganic heat-transfer fluid is less than about 800, 600, 400, 300 joulesper gram of polyester melt product. More preferably, the organicheat-transfer fluid is heated by steam heat-transfer fluid according tothe disclosures herein. It is preferred that the thermal energy suppliedto a polyester production facility by low-vapor-pressure organicheat-transfer fluid is at least about 40, 80, 120, 160 joules per gramof polyester melt product. More preferably, the organic heat-transferfluid is heated by steam heat-transfer fluid according to thedisclosures herein.

In yet another embodiment of the invention, it is preferred thatcondensate-water is taken from at least one heat-exchange means that isusing steam heat-transfer fluid operating according to at least oneembodiment herein and that the condensate-water is reduced in pressureto form lower pressure flashed-steam heat-transfer fluid. The means forconverting the condensate-water to the flashed-steam can be locatedeither inside or outside the polyester production facility. It ispreferred that at least a portion of the flashed-steam heat-transferfluid is formed having a pressure that is less than about 8, 7, 5, 2megapascals. It is preferred that at least a portion of theflashed-steam heat-transfer fluid is formed having a pressure that is atleast about 0.2, 0.6, 1.0, 1.5 megapascals. It is preferred that atleast a portion of the flashed-steam heat-transfer fluid is producedfrom condensate-water that is formed from steam heat-transfer fluid usedto heat reaction medium. It is preferred that at least a portion of theflashed-steam heat-transfer fluid is produced from condensate-water thatis formed from steam heat-transfer fluid used to heat alow-vapor-pressure organic heat-transfer fluid.

It is preferred that at least a portion of the flashed-steamheat-transfer fluid is used to provide energy to a polyester productionfacility and/or other adjacent chemical synthesis facilities. It ispreferred that at least a portion of the flashed-steam heat-transferfluid is used within a polyester production facility to provide thermalenergy to process materials. More preferably, at least a portion of theflashed-steam heat-transfer fluid is used within a polyester productionfacility to form at least a portion of vaporized process material. Mostpreferably, at least a portion of the flashed-steam heat-transfer fluidis used within a polyester production facility to heat liquid EG to formvapor EG. Such a configuration is schematically illustrated in FIG. 4.

Additionally, it is preferred that at least a portion of theflashed-steam heat-transfer fluid is used in an adjacent terephthalicacid synthesis facility. An exemplary monomer production facility, suchas a terephthalic acid (TPA) synthesis facility, is schematicallydepicted in FIG. 5. The temperature of many major heat duties in amonomer production facility are usefully less than in a polyesterproduction facility, and this enables a favorable, efficient, energyintegration when steam heat-transfer fluid is used. The temperature ofthe reaction medium in a polyester production facility is preferably atleast about 250 degrees Celsius; and, as disclosed above, it ispreferred that the condensing temperature of the steam heat-transferfluid therein is at least about 260, 275, 290, 305 degrees Celsius.

FIG. 6 depicts one embodiment of a system for integrated steam heatingof a polymer production facility, such as a PET melt product synthesisfacility, and a monomer production facility, such as a TPA synthesisfacility. In particular, FIG. 6 shows how the heating duty ofhigh-pressure and lower-pressure steam can be divided up among thepolymer and monomer facilities.

In the monomer production facility, it is preferred that at least about30, 50, 70, 80 percent of the total thermal energy input from heattransfer fluids is to a process temperature of less than about 295, 280,265, 250 degrees Celsius. It is preferred that the terephthalic acidsynthesis facility is located such that the minimum horizontal distancefrom the polyester production facility is less than about 1800, 900,300, 100 meters. It is preferred that the terephthalic acid synthesisfacility forms at least a portion of terephthalic acid product bypartial oxidation of para-xylene, more preferably using molecularoxygen. It is preferred that at least a portion of the terephthalic acidproduct is fed into a reaction medium of the adjacent polyesterproduction facility within less than about 72, 24, 12, 4 hours afterbeing formed from para-xylene.

The invention may include various separate aspects, as set forth herein.Thus, in one aspect, the invention relates to a process for making apolyester in a polyester production facility, the process including thesteps of: forming a reaction medium comprising at least one monomer,wherein the at least one monomer comprises terephthalic acid (TPA)and/or an ester derivative of TPA; subjecting at least a portion of thereaction medium to one or more chemical reactions in the polymerproduction facility to thereby produce the polyester; and heating thereaction medium at one or more locations in the polyester productionfacility, wherein at least 30 percent of the total energy input employedfor the heating of the reaction medium is provided by indirect heatexchange between the reaction medium and steam. In other aspects, atleast 50, 70, or 80 percent of the total energy input employed for theheating of the reaction medium may be provided by indirect heat exchangewith steam.

In another aspect, the amount of the heating of the reaction mediumprovided by indirect heat exchange with steam may be at least 350, or atleast 700, or at least 900, or at least 1,200 joules per gram of thepolyester produced from the polyester production facility.

In yet another aspect, the polyester may be produced from the polyesterproduction facility at a rate of at least 100, or at least 200, or atleast 400, or at least 600 million kilograms per year.

In a further aspect, at least 30, or at least 50, or at least 70, or atleast 90 percent of the heating of the reaction medium provided steam isaccomplished by indirect heat exchange with steam having a pressure ofat least 2, or at least 5, or at least 6, or at least 7, or at least 9megapascals. In yet another aspect, at least 30, or at least 50, or atleast 70, or at least 80 percent of the total energy input employed forheating any stream, conduit, and/or equipment in the polyesterproduction facility is provided by indirect heat exchange with steam.

In one aspect, the process further includes a step of generatingpressurized steam in one or more boilers, wherein the pressurized steamdirectly or indirectly provides at least 30, or at least 50, or at least70, or at least 80 percent of the total heat energy input required bythe polyester production facility. The thermal energy input provided bythe pressurized stream, in turn, may be at least 200, or at least 500,or at least 1,000, or at least 1,500 joules per gram of the polyesterproduced from the polyester production facility.

In yet another aspect, the heating of the reaction medium by indirectheat exchange with steam is carried out in one or more shell-and-tubeheat exchangers. The reaction medium may, in turn, flow on the tube-sideof the shell-and-tube heat exchangers and the steam may flow on theshell-side of the shell-and-tube heat exchangers. The reaction mediummay, in turn, flow through the tube-side of the shell-and-tube heatexchangers at a superficial velocity of at least 0.7, or at least 1.4,or at least 3, or at least 5 meters per second, measured on atime-averaged and cross-sectional-area-averaged basis. The tubes of theshell-and-tube heat exchanger may, in turn, have an outside diameter ofless than 0.1, or less than 0.05, or less than 0.01, or less than 0.005,or less than 0.003 meters.

In further aspects, the at least one monomer may further comprise analcohol, and the alcohol may comprise a diol, for example ethyleneglycol.

In yet another aspect, the reaction medium may be maintained at atemperature greater than 250° C. during at least one of the chemicalreactions.

In a further aspect, the steam employed for the heating of the reactionmedium may have an initial temperature of at least 250° C.

In another aspect, the chemical reactions include esterification andpolycondensation.

In yet another aspect, at least 75 percent of the heating of thereaction medium by indirect heat exchange with steam may be used to heatthe reaction medium for the esterification reaction.

In a further aspect, the one or more chemical reactions may be carriedout in a plurality of serially connected reactors, wherein the seriallyconnected reactors include a first reactor, a second reactor, and athird reactor. In yet another aspect, each of the first, second, andthird reactors may have a respective first, second, and third heatexchanger associated therewith. In yet another aspect, each of thefirst, second, and third reactors may have a respective first, second,and third vapor-liquid separation zone associated therewith, and theheat exchangers and vapor-liquid separation zones associated with eachof the first, second, and third reactors may be located in separatevessels.

In yet another aspect, the first, second, and third heat exchangers maybe shell-and-tube heat exchangers with steam on the shell-side and thereaction medium on the tube-side.

In yet another aspect, the first vapor-liquid separation zone may belocated downstream of the first heat exchanger, the second vapor-liquidseparation zone may be located downstream of the second heat exchanger,and the third vapor-liquid separation zone may be located downstream ofthe third heat exchanger. In a further aspect, esterification may becarried out in the first reactor, and the reaction medium may have aconversion of at least 50, or at least 70, or at least 80, or at least90 percent upon exiting the first reactor. In yet another aspect,esterification is carried out in the second reactor, and the reactionmedium may have a conversion of at least 80, or at least 90, or least95, or at least 97 percent upon exiting the second reactor.

In a further aspect, the first reactor has a first heat exchangerassociated therewith, wherein at least 20, or at least 40, or at least50, or at least 60 percent and/or up to 100, or up to 90, or up to 80,or up to 70 percent of the total energy input employed for the heatingof the reaction medium is provided by indirect heat exchange with steamin the first heat exchanger.

In yet another aspect, the amount of the heating of the reaction mediumprovided by indirect heat exchange with steam in the first heatexchanger may be at least 200, or at least 500, or at least 800, or atleast 1,000 joules per gram of the polyester produced from the polyesterproduction facility and/or up to 10,000, or up to 5,000, or up to 3,000,or up to 2,000 joules per gram of the polyester produced from thepolyester production facility.

In a further aspect, the amount of the heating of the reaction mediumprovided by indirect heat exchange with steam in the first heatexchanger may be at least 5, or at least 10, or at least 15, or at least25 megawatts.

In yet another aspect, the second reactor has a second heat exchangerassociated therewith, wherein at least 0.5, or at least 1, or at least2, or at least 4 percent and/or up to 50, or up to 30, or up to 15, orup to 10 percent of the total energy input employed for the heating ofthe reaction medium may be provided by indirect heat exchange with steamin the second heat exchanger. In another aspect, the amount of theheating of the reaction medium provided by indirect heat exchange withsteam in the second heat exchanger may be at least 20, or at least 50,or at least 80, or at least 100 joules per gram of the polyesterproduced from the polyester production facility and/or up to 1,000, orup to 500, or up to 300, or up to 200 joules per gram of the polyesterproduced from the polyester production facility.

In yet another aspect, the third reactor has a third heat exchangerassociated therewith, wherein at least 0.05, or at least 0.1, or atleast 0.5, or at least 1 percent and/or up to 30, or up to 20, or up to10, or up to 5 percent of the total energy input employed for theheating of the reaction medium may be provided by indirect heat exchangewith steam in the third heat exchanger. In a further aspect, the amountof the heating of the reaction medium provided by indirect heat exchangewith steam in the third heat exchanger may be at least 1, or at least 5,or at least 10, or at least 20 joules per gram of the polyester producedfrom the polyester production facility and/or up to 500, or up to 200,or up to 100, or up to 50 joules per gram of the polyester produced fromthe polyester production facility.

In a further aspect, polycondensation may be carried out downstream ofthe first, second, and/or third reactor.

In yet another aspect, the serially connected reactors may include afourth reactor and a fifth reactor, and polycondensation may be carriedout in the fourth and fifth reactors. In a further aspect, the polyesteris recovered from to fifth reactor.

In another aspect, esterification may be the predominate reaction in thefirst and second reactors and polycondensation may be the predominatereaction in the fourth and fifth reactors.

In yet another aspect, the temperature of the reaction medium exitingthe first, second, and/or third heat exchangers may be at least 250° C.

In yet another aspect, at least 1, or at least 3, or at least 5, or atleast 10 percent and/or up to 75, or up to 50, or up to 30, or up to 20percent of the total energy input employed for the heating of thereaction medium may be provided by indirect heat exchange with a heattransfer medium (HTM) that is not steam. In a further aspect, the amountof the heating provided by indirect heat exchange with the HTM may be atleast 1, or at least 5, or at least 10, or at least 20 joules per gramof the polyester produced from the polyester production facility and/orup to 500, or up to 250, or up to 100, or up to 50 joules per gram ofthe polyester produced from the polyester production facility.

In yet another aspect, the process further comprises heating the HTM byindirect heat exchange with steam.

Example

This is a calculational example that quantifies the heat duty associatedwith PET melt product synthesis facilities. The table below sets forthcalculated values for the heat duties of a PET melt product synthesisfacility constructed in accordance with the inventive embodimentdepicted in FIG. 2, where all heat duties are supplied by steam.

BTU/ BTU/hr lb PET % J/g MW PET Melt Synthesis HTM S&T Ex 24,000,000 13113.9% 304 7.0 Rx#1 S&T Ex 115,400,000 628 67.0% 1,461 33.8 Rx#2 S&T Ex10,500,000 57 6.1% 133 3.1 Rx#3 S&T Ex 3,000,000 16 1.7% 38 0.9 EGvapor, vacuum jets 14,127,000 77 8.2% 179 4.1 EG Water Column 5,130,00028 3.0% 65 1.5 total PET 172,157,000 937 100.0% 2,180 50.5 Rxn medium,including ambient loss HTM S&T Ex 24,000,000 131 15.7% 304 7.0 3 Rx Ex128,900,000 702 84.3% 1,632 37.8 total PET 152,900,000 832 100.0% 1,93644.8 83,307 kg/hr PET 183,692 lb/hr PET 350 days/yr 700,000,000 kg/yrPET HTM = Heat Transfer Medium = low-vapor-pressure organicheat-transfer fluid S&T Ex = Shell and Tube Exchanger Rx = Reactor

In a conventional PET melt product synthesis facility, at least thefirst four heating duties, which sum to 152,900,000 BTU/hr (1,936 joulesper gram; 44.8 megawatts), would be provided by a fuel-fired furnaceheating low-vapor-pressure organic heat-transfer fluid (HTM).

1. A process for making a polyester in a polyester production facility,the process comprising: (a) forming a reaction medium comprising atleast one of terephthalic acid (TPA) and an ester derivative of TPA, andat least one diol; (b) reacting at least a portion of the reactionmedium in the polymer production facility to thereby produce thepolyester; and (c) heating the reaction medium during the reacting atone or more locations in the polyester production facility, wherein atleast 50 percent of the total energy input employed for the heating ofthe reaction medium is heat exchange between the reaction medium andsteam in a heat exchanger containing the reaction medium and the steam.2. The process of claim 1, wherein at least 80 percent of the totalenergy input employed for the heating of the reaction medium is heatexchange between the reaction medium and steam.
 3. The process of claim1, wherein the amount the heating of the reaction medium provided by thesteam is at least 900 joules per gram of the polyester produced from thepolyester production facility.
 4. The process of claim 1, wherein thepolyester is produced from the polyester production facility at a rateof at least 400 million kilograms per year.
 5. The process of claim 1,wherein the heating of the reaction medium by the steam is carried outin one or more shell-and-tube heat exchangers.
 6. The process of claim5, wherein the reaction medium flows on the tube-side of the one or moreshell-and-tube heat exchangers and steam flows on the shell-side of theone or more shell-and-tube heat exchangers.
 7. The process of claim 5,wherein the reaction medium flows through the tube-side of theshell-and-tube heat exchangers at a superficial velocity of at least 1.4meters per second, measured on a time-averaged andcross-sectional-area-averaged basis.
 8. The process of claim 5, whereinthe tubes of the shell-and-tube heat exchangers have an outside diameterof less than 0.003 meters.
 9. The process of claim 1, wherein at least50 percent of the heating of the reaction medium provided by the steamis accomplished by heat exchange with steam having a pressure of atleast 7 megapascals.
 10. The process of claim 1, wherein the reactionmedium comprises ethylene glycol.
 11. The process of claim 1, whereinthe reaction medium is maintained at a temperature greater than 250° C.during the reacting.
 12. The process of claim 1, wherein the steamemployed for the heating of the reaction medium has an initialtemperature of at least 250° C.
 13. The process of claim 1, wherein thereacting comprises esterification and polycondensation.
 14. The processof claim 13, wherein at least 75 percent of the heating of the reactionmedium by the steam is used to heat the reaction medium for anesterification reaction.
 15. The process of claim 1, wherein thereacting is carried out in a plurality of serially connected reactors,wherein the serially connected reactors include a first reactor, asecond reactor, and a third reactor.
 16. The process of claim 15,wherein each of the first, second, and third reactors has a respectivefirst, second, and third reactor heat exchanger associated therewith.17. The process of claim 16, wherein the first, second, and thirdreactor heat exchangers are shell-and-tube heat exchangers with steam onthe shell-side and the reaction medium on the tube-side.
 18. The processof claim 16, wherein each of the first, second, and third reactors has arespective first, second and third vapor-liquid separation zoneassociated therewith, and wherein the reactor heat exchangers andvapor-liquid separation zones associated with each of the first, second,and third reactors are located in separate vessels.
 19. The process ofclaim 17, wherein esterification is carried out in the first reactor,and wherein the reaction medium has a conversion of at least 70 percentupon exiting the first reactor.
 20. The process of claim 16, whereinesterification is carried out in the second reactor, and wherein thereaction medium has a conversion of at least 90 percent upon exiting thesecond reactor.
 21. The process of claim 16, wherein the amount of theheating of the reaction medium provided by the steam in the firstreactor heat exchanger is from 500 joules per gram to 3,000 joules pergram of the polyester produced from the polyester production facility.22. The process of claim 16, wherein the serially connected reactorsinclude a fourth reactor and a fifth reactor, and whereinpolycondensation is carried out in the fourth and fifth reactors. 23.The process of claim 22, wherein the polyester is recovered from thefifth reactor.
 24. The process of claim 22, wherein esterification isthe predominate reaction in the first and second reactors andpolycondensation is the predominate reaction in the fourth and fifthreactors.
 25. The process of claim 1, wherein from 1 percent to 50percent of the total energy input employed for the heating of thereaction medium is provided by heat exchange with a heat transfer medium(HTM) that is not steam.
 26. The process of claim 25, wherein the HTM isheated by heat exchange with steam.
 27. The process of claim 1, whereinthe (b) reacting and the (c) heating are carried out in the same vessel.